Alpha-1 antitrypsin (AAT) deficiency is an autosomal recessive genetic disorder causing both lung and liver diseases, for which there is no effective treatment. The most common genotype of AAT deficiency is genotype PiZZ, which encodes mutant AAT, referred to as Z protein (ATZ). It affects 1 in 1,800 live births in Northern European and North American populations. The fundamental pathological process of the AAT deficiency is the accumulation of mutant AAT as polymers within hepatocytes. The resultant low levels of AAT in the serum, result in lung damage by proteinases, and eventually emphysema. Though not entirely clear, the protein accumulation in hepatocytes appears to play a crucial role in the development of liver diseases, including cirrhosis and hepatocellular carcinoma.
AAT is a member of the serine proteinase inhibitor family1. Its main function is to protect tissue from the damage caused by various proteinases during inflammatory responses2, 3. The liver is the main source of AAT. Deficiency in AAT causes both lung and liver diseases4. There is no effective treatment available, except for symptomatic control and replacement therapy5. A current focus on new treatment modalities is gene therapy6. Although gene therapy may alleviate lung disease, the liver disorders are expected to remain. The ideal treatment would be a therapeutic intervention that promotes ATZ secretion from hepatocytes, which could cure both lung and liver diseases, and probably other diseases that are associated with AAT deficiency, such as panniculitis, vasculitis, pancreatitis or renal disease7.
The prototype of AAT deficiency (PiZZ) affects 1 in 1,800 live births in Northern European and North American populations8, 9. The disease is associated with mutation of the gene, AAT10. The Z form of AAT is a mutation that results from the substitution of lysine for glutamate at position 342, and accounts for the defective secretion and mutant molecule accumulation in the endoplasmic reticulum of hepatocytes11-14. In ZZ homozygotes, the low serum level of AAT predisposes the patients to lung disease, such as emphysema. In a subgroup of AAT deficiency patients, liver diseases also occur, which include chronic hepatitis, cirrhosis, and hepatocellular carcinoma15. In fact, AAT deficiency-associated liver disease is the most common genetic liver disease in children and the most common genetic diagnosis for liver transplantation16. However, the pathogenesis of the liver disease is poorly understood.
The lung disease in AAT deficiency patients is usually of an earlier onset than in patients with chronic obstructive pulmonary disease (COPD) and often appears to be out of proportion to their smoking history. The typical pattern shows lower lobe predominant or pan-lobular emphysema17, 18. The pathogenesis of emphysema associated with AAT deficiency is closely related to neutrophil elastase. Leucocyte elastase, a neutrophil enzyme, can bind to the active site of AAT and permanently inactivates it, which causes elastin degradation, and lung tissue injury and destruction19, 20 Smoking is a definite compounding factor for the development of lung disease. Other genetic factors and environmental conditions are also implicated in the pathogenesis of AAT-associated lung disease21.
The current concept for AAT deficiency-associated liver cell injury is “gain of function”16, 22. In another words, it is related to the protein accumulation within hepatocytes (and, hence, is actually a storage disorder). The supporting evidence was mainly derived from studies using mice transgenic for mutant human AAT23-25. Although the detailed kinetics of mutant AAT within a hepatocyte are still not completely defined, several groups have demonstrated that the mutation of AAT affects the gap between the third and fifth strands of the “A” sheet of the protein, which results in dimerization26-28. The dimerized proteins eventually form polymers, which are retained in the ER. It is also possible that some unidentified cellular factors play a role in the turnover of the mutant AAT, though the details are still unknown. Recent studies have shown that chemical chaperons can reverse the cellular mislocalization or misfolding of mutant protein29, 30. It has been shown that 4-phenylbutyric acid (PBA) can increase blood levels of AAT in a human ATZ transgenic model29. Its potential clinical effectiveness is currently undergoing evaluation).
It is still unclear how the retained protein causes cell damage. Recent studies by Teckman et al. suggested that the accumulation could initiate cellular responses31, 32. Among the responses is increased numbers of autophagosomes31, 32. It is known that autophagy is associated with cell stress, differentiation and morphogenesis33. The autophagic response in the hepatocytes with mutant AAT is probably a protective mechanism for host cells. Interestingly, both AAT and mitochondria are present in the autophagosomes31. Moreover, the mitochondria that are not surrounded by the autophagic vacuolar membranes are invariably damaged to a certain extent, indicating mitochondria may be involved in mutant AAT associated liver cell injury. Many studies have attributed the mitochondrion as one of the key players regulating program cell death (apoptosis)34, 35. Therefore, a working hypothesis is that the accumulation of mutant AAT may subject the host cell undergoing apoptosis through signaling pathways related to the mitochondria. Supporting this notion, Perlmutter et al. have shown that activated caspase-3 is increased in the ATZ mouse liver16. These observations indicate that hepatocyte apoptosis may be an important mechanism for ATZ-related liver damage.
Clinical studies have indicated that the protein accumulation alone could not explain all the cases of the liver diseases, implying that other factors may play a role in the pathogenesis, such as environmental factors and genetic traits16, 36, 37. It has been shown that increasing ambient temperature causes an increase in the polymerization of mutant AAT11. The phenomenon has been employed as an in vitro assay to study biochemical mechanisms of AAT polymerization. Systemic diseases also affect liver disease incidence and severity, probably through cytokines. It is known that several cytokines such as IL-1 or IL-6, affect expression levels of AAT38-40. However, little is known on how these cytokines are involved in the disease process. In the case of IL-6, its signal is transmitted through STAT3 (signal transducer and activator of transcription 3). The binding site of STAT3 has been identified in the enhancer region of the AAT gene41.
It appears that mutant AAT retention through polymerization is a key mechanism of hepatocyte damage. It is also the cause for low level of AAT in the serum.
Except for symptomatic control and replacement therapy, there is currently no effective treatment available for AAT deficiency.